AVIONICS CORROSION PROGRAM AND CORROSION THEORY

7/25/2019 AVIONICS CORROSION PROGRAM AND CORROSION THEORY

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TECHNICAL MANUAL

CLEANING AND CORROSIONCONTROL

VOLUME ICORROSION PROGRAM AND

CORROSION THEORY

01 MARCH 2005

This publication supersedes NAVAIR 01-1A-509/TM 1-1500-344-23, dated 1 May 2001and NAVAIR 16-1-540/TM 1-1500-343-23/TO 1-1-689, dated 1 Sep 2000.

DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited.

DESTRUCTION NOTICE - For unclassified, limited documents, destroy by any method that will prevendisclosure of contents or reconstruction of the document.

PUBLISHED BY DIRECTION OF COMMANDER, NAVAL AIR SYSTEMS COMMAND

0801LP1043459

NATEC ELECTRONIC MANUAL

Downloaded from http://www.everyspec.com on 2012-02-20T1:25:01.


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LIST OF EFFECTIVE PAGES

Dates of issue for original and changed pages are:

Original........................ 0 ......................... 01 Mar 2005Change ....................... x ........................ xx XXX 199X

Insert latest changed pages; dispose of superseded pages in accordance with applicable regulations.

NOTE: On a changed page, the portion of the text affected by the latest change is indicated be a vertical l ine, orother change symbol in the outer margin of the page. Change in illustrations are indicated by miniature pointinghands. Changes to wiring diagrams are indicated by shaded areas.

Total number of pages in this manual is 52, consisting of the following:

Page *Change Page *Change Page *ChangeNo. No. No. No. No. No.

Change ....................... 0 ......................... 15 Sep 1993Change ....................... x ........................ xx XXX 199X

A Change X

*Zero in this column indicates an original page.

Title ........................................0A .............................................0i-ii ...........................................0TPDR-1 ..................................0TPDR-2 Blank ........................01-1 - 1-3 ................................. 01-4 Blank ................................02-1 - 2-4 ................................. 03-1 - 3-25 ...............................03-26 Blank ..............................0Glossary-1 - Glossary-8 ......... 0Index-1 - Index-3 ....................0Index-4 Blank ......................... 0



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LIST OF ILLUSTRATIONS ......................................... ii

LIST OF TABLES ........................................................ iiLIST OF TECHNICAL PUBLICATIONSDEFICIANCE REPORTS (TPDR)INCORPORATED ............................TPDR-1

1 INTRODUCTION

1-1. Overview ......................................... 1-11-2. Purpose ........................................... 1-11-3. Scope .............................................. 1-11-4. Arrangement of Manual .................. 1-11-5. Related Publications ....................... 1-21-6. Usage and Coflicts .......................... 1-2

1-7 Reporting Errors and ImprovementRecommendations........................ 1-2

1-8. Manual Change Procedures ........... 1-31-9. Requisitioning and Automatic

Distribution ................................... 1-3

2 PREVENTATIVE MAINTENANCEPROGRAM

2-1. Overview ......................................... 2-12-2. Corrosion Prevention Philosophy ... 2-12-3. Preventive Maintenance ................ 2-12-4. Aircraft Preventive

Maintenance Program .................. 2-12-5. Avionics Preventive

Maintenance Program .................. 2-2

2-6. Corrosion Control Program ............. 2-2

2-7. Corrosion-Related FailureData Feedback ............................. 2-32-8. Safety .............................................. 2-42-9. Materials ......................................... 2-4

3 CORROSION THEORY

3-1. Overview ......................................... 3-13-2. Purpose ........................................... 3-13-3. Scope .............................................. 3-13-4. Definition of Corrosion .................... 3-13-5. Chemical Definitions ....................... 3-13-6. Theory of Corrosion ........................ 3-1

3-7. Development of Corrosion .............. 3-23-8. Factors Influencing Corrosion ......... 3-33-9. Types of Corrosion.......................... 3-63-10. Metals Affected by Corrosion........ 3-113-11. Degradation of Non-Metals ........... 3-163-12. Effects of Environment on

Corrosion .................................... 3-183-13. Natural Environment ..................... 3-193-14. Biological Corrosion ...................... 3-213-15. Man-Made Environments .............. 3-23

GLOSSARY ................................................ Glossary-1ALPHABETICAL INDEX .................................. Index-1

TABLE OF CONTENTS

Chapter Page Chapter Page



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LIST OF TABLES

Table Title Page Table Title Page

1-1. Outline of Manual - All Volumes .................. 1-11-2. Outline of Volume I ....................................... 1-2

3-1. Effects of Corrosion on Metals.................... 3-133-2. Effects of Deterioration on NonMetals ........ 3-17

Figure Title Page Figure Title Page

LIST OF ILLUSTRATIONS

2-1. Corrosion Prevention Program ..................... 2-1

2-2. Basic Maintenance Functions ....................... 2-3

3-1. Simplified Corrosion Cell............................... 3-23-2. Elimination of Corrosion by Application

of an Organic Film to a Metal Surface....... 3-23-3. Effect of Sea Water on

Galvanic Corrosion .................................... 3-23-4. Galvanic Series of Metals and

Alloys in Sea Water ................................... 3-43-5. Galvanic Corrosion in a Flashlight Battery ... 3-53-6. Effect of Area Relationship in

Dissimilar Metal Contacts .......................... 3-53-7. Surface Corrosion on Frequency Test Set ... 3-6

3-8. Galvanic Corrosion of MagnesiumAdjacent to a Steel Fastener ..................... 3-6

3-9. Variations in the Cross-SectionalShape of Corrosion Pits ............................ 3-6

3-10. Pitting of an Aluminum Wing Assembly ........ 3-73-11. Cross-Section of 7075-T6 Aluminum Alloy ... 3-73-12. Scanning Electron Micrograph of a

Corroding Aluminum Surface .................... 3-7

3-13. Intergranular Corrosion of

7075-T6 Aluminum Adjacent toSteel Fastener ........................................... 3-8

3-14. Extreme Example of Exfoliation atEdge of Sheet ............................................ 3-8

3-15. Exfoliation Adjacent to Fasteners ................. 3-83-16. Crevice Corrosion Mechanisms .................... 3-93-17. Filiform Corrosion Found Under Paint

Coating on a Magnesium Panel ................ 3-93-18. Schematic of the Development of Filiform

Corrosion on an Aluminum Alloy ............... 3-93-19. Cracking (Typical of Stress Corrosion or

Corrosion Fatigue) ................................... 3-103-20. Fretting Corrosion ....................................... 3-11

3-21. Hot Corrosion on Fasteners ........................ 3-123-22. Hot Corrosion on Engine Components ....... 3-123-23. Aluminum Surface Corrosion Products....... 3-133-24. Magnesium Corrosion Products.................. 3-133-25. Steel Corrosion Products ............................ 3-143-26. Color Changes in Titanium Due to

Heating .................................................... 3-143-27. Cadmium Plated Surface Conditions .......... 3-153-28. Failed Chromium Plate ............................... 3-153-29. Corroded Circuit Card ................................. 3-163-30. Biological Growth on Helicopter Wall .......... 3-21

3-3. Effects of Moisture and Fungi on VariousMaterials .................................................. 3-22

3-4. Effects of Airframe Flluid Intrusion.............. 3-24



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NONE

Report Control Number (RCN) Location

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Report Control Number (RCN) Location

AIMD NAS PENSACOLA, FL52814-2000-0022 Pg 9-2








LIST OF TECHNICAL PUBLICATIONS DEFICIENCY REPORTS INCORPORATED

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CHAPTER 1INTRODUCTION

1-1. OVERVIEW.Prevention and repair of corrosiondamage to aircraft and avionic systems continues to bean ever increasing cost and safety burden for militaryaircraft. Equipment is routinely exposed to changes intemperature and pressure, varying humidity levels,dust, dirt, ultraviolet light, aircraft fluids, and environmentsthat promote corrosion. Increasing environmental andsafety restrictions, which limit traditional corrosion controlmaterials, are also a significant factor in the safe andeconomic operation of aircraft and avionics.

1-1.1. The Cleaning and Corrosion Control manual

was established jointly by the Navy, Air Force, and Armyas a combined effort to consolidate and coordinatecorrosion control best practices for aircraft and avionics.

1-1.2. This volumized set of corrosion manualscombines and replaces the former Aircraft WeaponsSystems Cleaning and Corrosion Control (NAVAIR 01-1A-509/TM 1-1500-344-23) and Avionics Cleaning andCorrosion Prevention/Control (NAVAIR 16-1-540/TO1-1-689/TM 1-1500-343-23) manuals.

1-2. PURPOSE. The purpose of this manual is toprovide information on materials and procedures to

prevent, control, and repair corrosion damage to aircrafand avionics on land or at sea.

1-3. SCOPE. The material in this manual containsbasic corrosion prevention and corrective maintenanceinformation to be used at Organizational, Intermediateand Depot levels.

1-4. ARRANGEMENT OF MANUAL.

1-4.1. OVERVIEW OF ALL VOLUMES. The NAVAIR01-1A-509/TM 1-1500-343-23/TO 1-1-689 series o

manuals is arranged as shown in Table 1-1.

1-4.1.1. A complete set of manuals to perform aircrafcleaning and corrosion control functions consists oVolumes I, II, and IV (replaces NAVAIR 01-1A-509TM 1-1500-344-23).

1-4.1.2. A complete set of manuals to perform avionicsand electronics cleaning and corrosion control functionsconsists of Volumes I, III, and IV (Navy and Army) oVolumes I, I I I , and V (Air Force) (replacesNAVAIR 16-1-540/TM 1-1500-343-23/TO 1-1-689).

This volume was prepared under the technical cognizance of theMaterials Engineering Division, NAVAIR North Island, San Diego, California.

Table 1-1. Outline of Manual - All Volumes

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.tiezingocer

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VI tnempiuqEdnaslairetaMelbamusnoCscinoivAdnatfarcriArof

gninaelcroftnempiuqednaslairetamdevorppastsilemulovsihT.noitamrofnigniredrosedivorpdna,lortnocnoisorrocdna

V tnempiuqEdnaslairetaMelbamusnoCscinoivArof

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.ylnoscinoivarof,noitamrofnigniredrosedivorpdna,lortnoc



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1-4.2. ARRANGEMENT OF VOLUME I. Volume Iconsists of three chapters and a glossary, arranged asshown in Table 1-2.

1-5. RELATED PUBLICATIONS.A listing of relatedpublications is provided in Chapter 1 of each volume ofthis manual, as applicable.

1-6. USAGE AND CONFLICTS.

1-6.1. Supervisory and maintenance personnel shalluse this manual as a guide for all corrosion control andmaintenance efforts. Contractors who maintain andrepair corrosion for military aircraft and avionics shallalso comply with the requirements of this manual.

1-6.2. This manual shall be used in conjunction with

and in support of the appropriate Army TechnicalManuals (TMs), Technical Bulletins (TBs), Departmentof the Army Pamphlets (DA PAMs), Navy MaintenanceInstruction Manuals (MIMs), Navy Structural RepairManuals (SRMs), Maintenance Requirement Cards(MRCs), or Air Force Technical Orders (TOs).

1-6.2.1. In the case of a conflict between this manualand other Navy manuals, this manual shall takeprecedence; however, maintenance activities shallcontact the appropriate Cognizant Field Activity (CFA)/Fleet Support Team (FST) for immediate resolution ofthe conflict.

1-6.2.2. The Army and Air Force specific systems/components manuals shall take precedence over thismanual.

1-6.3. WORDING. The following definitions are adheredto in preparing this manual.

1-6.3.1. Shall is used when a procedure is mandatory.

1-6.3.2. Should is used when a procedure isrecommended but not mandatory.

1-6.3.3. Will indicates future action but does not indicate

a mandatory procedure.

1-6.3.4. May is used only when a procedure is optional.

1-6.4. SYMBOLS (WARNINGS, CAUTIONS andNOTES). The following definitions apply to WARNINGS,CAUTIONS and NOTES found throughout the manual.

1-6.4.1. WARNING. An operation or maintenanceprocedure, practice, condition, or statement, which ifnot strictly observed, could result in injury to or death ofpersonnel, or long term health hazards to personnel.

1-6.4.2. CAUTION. An operating or maintenanceprocedure, practice, condition, or statement, which ifnot strictly observed, could result in damage/destructionof equipment or loss of mission effectiveness.

1-6.4.3. NOTE. An operating procedure, practice, orcondition which is essential to emphasize.

1-6.5. SERVICE DESIGNATIONS. Since this is a tri-service manual, not all sections apply to all services.Information within the text that does not apply to all threeservices is designated after the paragraph number asfollows: (N) NAVY ONLY, (A) ARMY ONLY, or (AF) AIRFORCE ONLY. Large sections that are service specificare included as appendices in the appropriate volume.

1-7. REPORTING ERRORS AND IMPROVEMENTRECOMMENDATIONS.

1-7.1. GENERAL. All activities using this manual areinvited to submit recommended changes, additions, ordeletions.

Table 1-2. Outline of Volume I

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yrassolG dnatfarcriagnimrofreplennosrepybdesuylnommocsmretsenifedyrassolgehT.lortnocnoisorrocdnagninaelcscinoiva



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1-7.2. SPECIFIC REPORTING REQUIREMENTS.Recommended changes, additions, or deletions shallbe reported as follows:

1-7.2.1. Navy personnel should submit recommended

changes to the appropriate technical services facilityusing the reporting system outlined in OPNAVINST4790.2.

1-7.2.2. Air Force personnel should refer to TO 00-5-1to report changes.

1-7.2.3. Army personnel should submit completed DA2028/2028-2 forms to Commander, U.S. Army Aviationand Missile Command, ATTN: AMSAM-MMC-MA-NP,Redstone Arsenal, AL 35898-5220. Changes may alsobe submitted electronically via the Army website,https://amcom2028.redstone.army.mil, or via email to:

2028@redstpme/army.mil.

1-8. MANUAL CHANGE PROCEDURES.

1-8.1. RESPONSIBILITY. This manual is a tri-servicedocument, coordinated by the Materials EngineeringDivision, Naval Air Depot North Island, Code 4.9.7,San Diego, CA. The following activities are responsiblefor maintaining this document: the Naval Air SystemsCommand, the Air Force Corrosion Program Office,and the U.S. Army Aviation and Missile Command. Asnecessary, representatives from these activities shallmeet to review proposed engineering and logistical

changes to this manual. Changes are approved by allservices, except for service-specific information.

1-8.2. PROCEDURES. The Navy is the lead servicefor publication of this manual; therefore, the followingNavy publication change procedures apply:

1-8.2.1. Revisions. Volumes will be updated periodicallyby the issuance of a revision, which is a completereplacement of all pages with all change informationincorporated.

1-8.2.2. Routine Changes. Between revisions, routinechanges may be issued in the form of corrected pages

to a portion of the existing manual. They consist oreplacement change pages for that section of the manuaaffected by the change.

1-8.2.3. Rapid Action Changes. Changes may be

issued as a formal Rapid Action Change (RAC) or anInterim Rapid Action Change (IRAC). IRACs are issuedas naval messages to expedite the release of urgenand essential operational and maintenance changeinformation. Army and Air Force program managers areresponsible for retransmittal of IRACs to the appropriateservice addressees.

1-9. REQUISITIONING AND AUTOMATICDISTRIBUTION.

1-9.1. Procedures to be used by Naval activities andother Department of Defense activities requiring NAVAIR

technical manuals are defined in NAVAIR 00-25-100.

1-9.2. To automatically receive future changes andrevisions to NAVAIR technical manuals, an activitymust be established on the Automatic DistributionRequirements List (ADRL) maintained by the Naval AiTechnical Data and Engineering Service Command(NATEC). To become established on the ADRL, notifyyour activity central technical publications librarian. Iyour activity does not have a l ibrary, you may establishyour automatic distribution requirements by contactingthe Commanding Officer, NATEC, Naval Air StationNorth Island, P.O. Box 357031, Bldg. 90 Distribution

San Diego, CA 92135-7031. Annual reconfirmation othese requirements is necessary to remain on automaticdistribution. Use your NATEC assigned account numbewhenever referring to automatic distr ibutionrequirements.

1-9.3. If additional or replacement copies of this manuaare required with no attendant changes in the ADRLthey may be ordered by submitting requisitions to theCommanding Officer, Naval Supply Systems CommandNaval Logistics Library, 5801 Tabor AvenuePhiladelphia, PA 19120-5099.

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CHAPTER 2PREVENTIVE MAINTENANCE PROGRAM

2-1. OVERVIEW.Investigations during the past tenyears have identified corrosion as a major factor in

electronics failure in the field. As much as 30% to 40%of military avionic failures are due to the corrosionprocess. This is despite steady improvements in reliabilityof avionic systems fielded to date and outlines the needfor an effective preventive maintenance program.

2-2. CORROSION PREVENTION PHILOSOPHY.Corrosion and environmental conditions are naturalphenomena that adversely affect equipment in fieldservice. Although never totally eliminated, the problemsthese factors cause can be minimized so that they areless severe and better controlled. This can be achievedby understanding equipment failure mechanisms and

development/utilization of corrosion control technology.

2-2.1. As a general rule, maintenance personnel shouldassume corrosion is ongoing, regardless of visiblephysical evidence. The aim of corrosion prevention is toenable systems to perform satisfactorily for a specifiedtime period. In other words, maintenance efforts shouldallow equipment to approach its maximum li fetime.

2-2.2. The general workflow diagram, in Figure 2-1,defines procedures followed to implement a corrosionpreventive maintenance program. This process isdesigned to indicate the sequence of events needed to

implement and maintain an effective corrosionprevention and control program.

2-3. PREVENTIVE MAINTENANCE.The two mostimportant factors in preventing corrosion, and the onlyones which can be controlled by field personnel, are theremoval of the electrolyte and the application of protectivecoatings. Since the extent of corrosion depends on thelength of time electrolytes are in contact with metals,aircraft corrosion can be minimized by frequent washing.If noncorrosive cleaners are used, the more frequentlya surface is cleaned in a corrosive environment the less

the possibility of corrosive attack. In addition, bymaintaining chemical treatments and paint finishes ingood condition, corrosion can be minimized. Thedegradation of non-metallic materials can be minimizedby avoiding the use of unauthorized maintenancechemicals and procedures. In addition, when repair orreplacement of non-metallic materials is required, onlyapproved materials shall be used. Dedication to properpreventive maintenance practices maximizes equipmentreliability.

2-4. AIRCRAFT PREVENTIVE MAINTENANCEPROGRAM.

2-4.1. The prevention and control of corrosion on aircrafand related equipment is a command responsibilityEach command must place special emphasis on theimportance of the corrosion control program and lend itsfull support to ensure that corrosion prevention andcontrol receives sufficient priority to be accomplishedalong with other required maintenance.

Figure 2-1. Corrosion Prevention ProgramGeneral Workflow Diagram

Train Personnel to Detect, Identify,

Clean, Preserve, Treat& Prevent Corrosion

(Re)Emphasize the Concept of All HandsResponsibility for Corrosion Control

Conduct Required MaintenanceCorrosion Inspections

Report AnyMaterial/Design Deficiencies

Treat Corrosion Promptly AfterDetection Using the Approved

Materials, Equipment & Techniques

Clean, Preserve, & LubricateEquipment at Prescribed Intervals

Maintain AccurateMaintenance Records

CorrosionDamage Present

?

NO

YES



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2-4.2. Aluminum and magnesium alloys found inaviation equipment will normally begin to corrode if saltdeposits, other corrosive soils, or electrolytes are allowedto remain. In order to prevent corrosion, a constantcycle of cleaning, inspection, operational preservation,

and lubrication must be followed. Prompt detection andremoval of corrosion will limit the extent of damage toaircraft components. An effective preventivemaintenance program requires cleaning, lubricationand preservation, as well as corrosion removal, paintremoval, surface treatment, sealing, and painting. Adisciplined preventive maintenance program includes:

a. Regularly scheduled aircraft washing as specifiedby parent service organization directives;

b. Using clean water with low chloride content foraircraft washing and rinsing (chloride content should be

less than 400 parts per million, approximately the samelimit as that for potable water);

c. Regularly scheduled cleaning or wipe down of allexposed unpainted surfaces, such as landing gearstruts and actuating rods of hydraulic cylinders asspecified by parent service organization directives, witha compatible fluid or lubricant;

d. Keeping low-point drains open;

e. Inspection, removal, and reapplication of corrosionpreventative compounds (CPCs) on a scheduled basis;

f. Earliest detection and repair of damagedprotective coatings; and

g. Using padded panel racks to store panels/partsfor aircraft and equipment during maintenance andusing protective measures to prevent abrasions/scratches resulting from placement of parts, tools, ortool boxes on wings, fuselage or other aircraft surfaces.

2-5. AVIONICS PREVENTIVE MAINTENANCEPROGRAM.

2-5.1. PROGRAM REQUIREMENTS. Successfulavionic cleaning and corrosion prevention/control effortsdepend on a coordinated, comprehensive preventivemaintenance program. Everyone involved inmaintenance, repair, and operation of avionic systemsmust be concerned with corrosion, cleaning, inspection,prevention, and treatment. Specifically, avionic corrosionprevention/control is everyones responsibility. Eachcommand must place special emphasis on the corrosion

control program and lend their full support. This ensuresthe program receives sufficient priority to beaccomplished along with other required maintenance.The goal of a preventive maintenance program is to haltcorrosion before significant decline in equipment

performance occurs. As such, it is important to recognizethe difference between prevention of corrosion andrepair of damage caused by corrosion. A preventivemaintenance program at the Organizational/Unit andIntermediate Maintenance Activities should:

a. Reduce the maintenance time spent repairingcorrosion damage.

b. Improve avionic system reliability, durability, andservice life.

c. Make the military avionics community aware of

the extent of the problem.

d. Report any deficiencies with materials andprocesses associated with corrosion control.

2-5.2. APPLICABLE GUIDELINES. All activitiesresponsible for the maintenance of military aircraft andavionic systems shall establish a corrosion prevention/control program. The type of program depends on theconditions or environments to which the aircraft/avionicsystems are exposed. Those aircraft and avionic systemsexposed to salt-air and tropical environments requirethe most stringent corrosion prevention and control

programs.

2-5.3. MAINTENANCE FUNCTIONS. Experience hasshown that all activities have a corrosion problem. Thisis regardless of whether the equipment is an installedavionic system, ground support equipment, or missilesystem. Accordingly, corrosion control efforts by allhands is mandatory. This must be a day-to-dayrequirement to prevent corrosion before it starts.Figure 2-2 depicts the basic maintenance functions.

2-6. CORROSION CONTROL PROGRAM. Allactivities responsible for aircraft maintenance shallestablish corrosion control programs as required by theparent service organization. This program shall bestructured as required by OPNAVINST 4790.2 (Navy),TO 00-20-1 (Air Force), or AR 750-59 (Army) andensure that personnel receive hazardous material/wastehandlers training. The type of program depends uponthe environment to which the aircraft may be exposed.At sea, where conditions are normally the most severe,aircraft are exposed to salt spray, ship stack gases, and



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aircraft engine exhausts. Land-based aircraft may be

exposed to industrial gases, salts, rain, mud, and, nearsalt water, mists containing sea salts. A comprehensivecorrosion control program shall consist of either aCorrosion Control Work Center or a Corrosion ControlTeam with personnel trained in the prevention, earlydetection, reporting, and repair of corrosion damage.Such a program requires a dedicated effort by allmaintenance personnel to prevent corrosion before itstarts. These efforts will improve the operationalreadiness of equipment and minimize costly repairs.

2-6.1. TRAINING. Personnel performing maintenanceon aircraft shall be trained in basic corrosion controskills as established by the parent service organizationPersonnel shall be fully aware of the reasons for thecorrosion control program. Without such training and

understanding, further damage or additional problemsmay result.

2-6.2. TRAINING AND QUALIFICATION REQUIREMENTS. Personnel responsible for corrosion contromaintenance and treatment shall receive the followingtraining.

a. Supervisors and corrosion control personnel shalattend basic corrosion control courses established bythe parent service organization.

b. Cleaning and repair personnel shall be trained in

inspection, identification, cleaning, treatmentpreservation, lubrication, hazardous material handling/hazardous waste disposal, and proper documentationreporting.

c. Supervisors shall ensure maximum use of inservice and on-the-job-training.

2-6.3. MAINTENANCE. An effective corrosion controprogram shall include thorough cleaning, inspectionpreservation, and lubrication, at specified intervals, inaccordance with Volumes II and III of this manualCheck for corrosion damage and integrity of protective

finishes during all scheduled and unscheduledmaintenance. Early detection and repair of corrosionwill limit further damage. When corrosion is discoveredtreat corrosion as prescribed in Volumes II and III assoon as possible and use only approved materialsequipment, and techniques. Only affected areas shalbe repaired. All maintenance personnel shall reporcorrosion promptly, in accordance with directivesestablished by the parent service organization.

2-7. CORROSION-RELATED FAILURE DATAFEEDBACK.

2-7.1. Since corrosion prevention and control for aircrafand avionics is a continuing concern, it is vitally importanthat corrosion problems are properly reported. Problemscan be corrected and improvements made to prevenreoccurrence in future equipment design. Identificationof the specific causes and extent of corrosion problemsis essential. Improved equipment performance andmaintenance assistance (personnel, equipmentmaterials, and procedures) are dependent on this data

Establish Maintenance Requirementsfor Corrosion Control

Establish Requirements

to Inspect for Corrosion Damage

Aquire Corrosion Control Equipment& Consumable Supplies

Establish Requirements forReporting Corrosion Damage

Develop & Maintain MaintenanceInstructions Outlining

Corrosion Control Program

Develop Program to Satisfy SpecificInspection, Cleaning, Treatment,

Preservation & Lubrication Requirements

Develop Cleaning & CorrosionTraining Program

Ensure Proper Training of Personnelin Corrosion Identification & Control

Conduct RequiredCorrosion Inspections

Ensure Cleaning, Treatment,Preservation, & Lubrication

are Completed

Figure 2-2. Basic Maintenance Functions



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2-7.2. MAINTENANCE AND READINESS DATACOLLECTION. All activities using this manual arerequired to use the current maintenance data collectionsystem(s) of the parent service organization. This willenable a record of corrosion-related failures to be

submitted to the appropriate technical services facilityfor analysis. Reporting personnel shall identify/reportcorrosion discrepancies in accordance with OPNAVINST4709.2 (Navy), TO 00-20-2 (Air Force), or DA PAM738-750/DA PAM 738-751 (Army).

2-8. SAFETY. Safety is everyones business andconcern.

2-8.1. RESPONSIBILITY OF SUPERVISORS.

2-8.1.1. Work center supervisors shall receive thefollowing training in accordance with parent service

directives:

a. The recognition and elimination of hazards;

b. Occupational safety and health;

c. The safety of the individual;

d. Accident investigation and reporting; and

e. The inspection and maintenance of personalprotective equipment (PPE).

2-8.1.2. Supervisors shall ensure that all corrosioncontrol personnel are informed of:

a. Current safety procedures;

b. Characteristics of materials to which they will beexposed; and

c. Required protective clothing to ensure safety ofpersonnel.

2-8.1.3 In addition, supervisors shall ensure that anadequate supply of safety equipment is in a ready-for-issue condition, and that the personnel under theircontrol are given, and use, appropriate protectiveequipment to prevent accidents, injuries, andoccupational illness.

2-8.2. RESPONSIBILITY OF PERSONNEL. Mainte-nance personnel shall use appropriate equipment whileexposed to hazardous conditions, and shall report tothe supervisor any protective equipment that is bro-

ken, damaged, defective, or inadequate. No one shalluse protective equipment that is not in a satisfactoryand serviceable condit ion. Personnel shall comply withoccupational safety and health requirements, includingmedical examinations, respirator training and fit testing,

and use of protection for eyes, ears, head, skin, and feet.

2-8.3. MATERIALS HANDLING. Many of the materialsand procedures outlined in this manual are potentiallyhazardous to personnel and potentially damaging toaircraft, especially with improper use. When using anychemicals, such as paint removers, detergents,conversion coatings, and solvents, follow the correctprocedures with appropriate protective gear to preventpersonnel injury and aircraft damage. Read theappropriate warnings and cautions in this manual priorto use of any hazardous materials. Misuse of certainmaterials can damage parts or cause corrosion which

may lead to catastrophic failure. Refer to DoD 6050.5-LR, Hazardous Materials Information System, or theappropriate parent service organization documents forthe handling, storage, and disposal of hazardousmaterials. Refer to local directives and policies pertainingto hazardous waste management. When in doubt,contact the local safety office, industrial hygienist,bioenvironmental engineer, or regional medical center.

2-9. MATERIALS.

2-9.1. Consumable materials and equipment listed inVolumes IV and V shall be used for corrosion control.

These materials and equipment have been approvedonly after extensive testing to prove their ability toperform properly and effectively without damaging anyof the metallic or nonmetallic materials used in aircraft.

2-9.2. Only those materials listed in this manual shallbe used for cleaning or corrosion control of aircraftcomponents. When several methods or materials arelisted, the preferred one is listed first, with alternatesfollowing. Materials listed in other manuals shall beused only when required procedures are not covered bythis manual. When approved materials are not available,substitutions shall only be made by the appropriateAircraft Controlling Custodians (ACC) or SystemProgram Manager (SPM).

2-9.3. Materials or processes considered to be animprovement over existing ones, after local laboratoryanalysis and evaluation, shall be forwarded to theAircraft Controlling Custodians (ACC) or SystemProgram Manager (SPM) for submission to the parentservice organization for further evaluation.



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CHAPTER 3CORROSION THEORY

3-1. OVERVIEW.

3-1.1. Maintenance of military aircraft and avionicequipment requires knowledge of why metals corrodeand materials degrade. The theory lies in the definitionand description of mechanisms that cause equipmentto fail in field service. Corrosion is the chemical orelectrochemical deterioration of a material. Thisdeterioration is complex in nature because of the varioustypes of corrosion, the frequent simultaneous presenceof several types of corrosion, and the designcharacteristics and maintenance/environmental factorsthat make aircraft and avionic systems susceptible tocorrosion.

3-1.2. Corrosion can cause complete failure ofequipment or undesirable changes in electricalcharacteristics. It is a process that is active on a 24 hourbasis. Equipment does not necessarily have to beinstalled, operated, or resident in a particularly harshenvironment. Some form of corrosion will take placeeven in near ideal environments. All personnel shouldrecognize that corrosion is the natural continuing processof materials returning to their normal state. Inadequatecorrosion prevention and control will ultimately affectequipment life cycles, downtime, and overall systemreliability.

3-2. PURPOSE. The purpose of this chapter is toprovide maintenance personnel with the backgroundknowledge necessary to understand the causes ofcorrosion.

3-3. SCOPE.This chapter is an introduction to corrosiontheory: the causes of corrosion and the factors whichinfluence its development. The theory of corrosion andthe factors influencing corrosion of aircraft metals aredescribed. The types of corrosion and how to recognizethem are discussed.

3-4. DEFINITION OF CORROSION.Corrosion is theelectrochemical deterioration of a material or itsproperties due to its chemical reaction with thesurrounding environment. This reaction occurs becauseof the tendency of metals to return to their naturallyoccurring state, usually oxide or sulfide ores. Forexample, iron in the presence of moisture and air willreturn to its natural state, iron oxide or rust. Aluminumand magnesium form corrosion products that are whiteoxides or hydroxides. When a water solution containingsoluble salts is present, corrosion of many alloys can

occur easily at ambient temperatures. This type ocorrosion can be effectively treated by maintenance

personnel as discussed in this manual. Corrosion canalso occur in the absence of water but only at hightemperatures, such as those found in gas turbineengines. However, the most common type of corrosion(and the one that can be most effectively treated bymaintenance personnel) is electrochemical corrosion.

3-5. CHEMICAL DEFINITIONS.

3-5.1. ATOM. The smallest unit of an element, madeup of a positively charged nucleus surrounded by asystem of negatively charged electrons. There are ove100 elements, including metals (such as aluminum

magnesium, gold, platinum, iron, nickel, titaniumcadmium, chromium, copper, silver, lead, berylliumzinc), and non-metals (such as carbon, boron, sulfurchlorine, hydrogen, oxygen, nitrogen, and helium).

3-5.2. ELECTRON. A negatively charged subatomicparticle. An electric current occurs when electrons areforced to move through metal conductors. Electronsflow through liquid solutions only in the presence oions.

3-5.3. ION. An atom or group of atoms or moleculeswhich has acquired a net electric charge by gaining

(negative ion) or losing (positive ion) electrons. Whenions are forced to move through liquid solutions, anelectric current can occur. Ions cannot move throughmetal conductors.

3-5.4. ELECTROLYTE. A liquid (usually water) solutioncontaining ions. Sea water is an electrolyte: an aqueous(water-based) solution whose major components aresodium and chloride ions. Electrochemistry is the branchof science concerned with chemical reactions at surfacesin contact with electrolytes.

3-6. THEORY OF CORROSION.All metals will corrodeto some extent in a natural environment. When a metacorrodes, the atoms lose electrons and become positivelycharged. In solution, the positively charged metal ionscan combine with negatively charged ions to formcorrosion products, such as metallic chlorides, oxideshydroxides, and sulfides.

3-6.1. Four conditions (illustrated in Figure 3-1) musexist before metal corrosion can occur.



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a. A metal which has a tendency to corrode mustbe present (the corroding metal is called the anode);

b. A dissimilar conductive material (the cathode),which has less tendency to corrode than the anode,must be present (a dissimilar metal may be a differentmetal, a protected part of the same metal, or conductiveplastic);

c. A conductive liquid (electrolyte) must connect the

anode and cathode (so that ions can carry electriccurrent between them); and

d. Electrical contact between the anode and cathode(usually in the form of metal-to-metal contact) mustexist so that electrons can move between the anodeand the cathode.

3-6.2. The elimination of any one of the four conditionswill slow or stop corrosion. For example, a paint film ona metal surface will prevent the electrolyte fromconnecting the anode and cathode, thereby stoppingthe electric current (see Figure 3-2). A change in the

electrolyte can also affect the rate of corrosion. Twoconnected dissimilar metal parts placed in distilledwater corrode very slowly due to a lack of ions in solutionto conduct the electric current; in sea water the corrosionreaction is accelerated by a factor of 1000 or more (seeFigure 3-3).

3-7. DEVELOPMENT OF CORROSION.All corrosiveattack begins on the surface of the metal. If allowed toprogress, corrosion can penetrate into the metal. If

Figure 3-1. Simplified Corrosion Cell

Electron Flow

Electrolyte(Fresh or Sea Water,

Acids, Gases)

AnodicArea

Metal

CathodicArea

Figure 3-3. Effect of Sea Water onGalvanic Corrosion

corrosion begins on an inside surface of a component(for example, the inner wall of a metal tube), it may goundetected until perforation occurs.

3-7.1. When corrosion products form, they oftenprecipitate onto the corroding surface as a powderydeposit. This film of corrosion products may reduce the

rate of corrosion, if the film acts like a barrier toelectrolytes. Some metals (such as stainless steel andtitanium), under the right conditions, produce corrosionproducts that are so tightly bound to the corroding metalthat they form an invisible oxide film (called a passivefilm) which prevents further corrosion. However, whenthe film of corrosion products is loose and porous (suchas those of aluminum and magnesium), an electrolytecan easily penetrate and continue the corrosion process,producing more extensive damage than surfaceappearance would indicate.

Figure 3-2. Elimination of Corrosion by Applicationof an Organic Film to a Metal Surface

AnodicArea

Metal

CathodicArea

UnbrokenPaint Film

No ContactBetween

Electrolyte &Anode &Cathode

Electrolyte(Continuous Liquid Path)



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3-7.2. Paint coatings can mask the initial stages ofcorrosion. Since corrosion products occupy more volumethan the original metal, the paint surfaces may becomeblistered, flaked, chipped, or appear lumpy.

3-8. FACTORS INFLUENCING CORROSION.Factorswhich influence metal corrosion and the rate of corrosionare outlined below.

3-8.1. TYPE OF MATERIAL. The best time to preventcorrosion is at the design stage. Proper material selectionis critical for the protection of equipment against harmfulenvironmental effects. Most pure metals are not suitablefor aircraft construction and are used only in combinationwith other metals, and sometimes non-metals, to formalloys. The metals most commonly used in aircraftconstruction are aluminum, steel, titanium, andmagnesium. Cadmium, nickel, chromium, and silver

are sometimes used as protective platings. Metals havea wide range of corrosion resistance. The most activemetals (those which tend to lose electrons easily), suchas magnesium and aluminum, corrode easily and arelisted at the top of Figure 3-4. The most noble metals(those which do not lose electrons easily), such as goldand silver, do not corrode easily and are listed at thebottom of Figure 3-4.

3-8.2. HEAT TREATMENT AND GRAIN DIRECTION.Most alloys are made up entirely of small crystallineregions called grains. When heat treated duringmanufacturing or repair, heavy sections of metals do

not cool uniformly and, as a result, tend to vary inchemical composition from one part of the metal toanother. This can cause corrosion if one area is moreactive than another. Alloys which are fabricated byrolling, extruding, forging, or pressing have propertieswhich depend highly on direction (parallel to grainelongation vs. cross grain). Corrosion can occur onsurfaces of those regions which are less resistant andalso at grain boundaries, resulting in the formation ofpits and intergranular corrosion. For example, exposedend grains corrode much more easily than flattenedelongated surfaces in sheet stock. This explains whyexfoliation occurs at the edge of aircraft skin sections ornext to countersunk fasteners.

3-8.3. DISSIMILAR METALS. When two dissimilarmetals make electrical contact in the presence of anelectrolyte, the rate at which corrosion occurs dependson the difference in their activities, that is, their positionsin Figure 3-4. The greater the difference in activity, the

faster corrosion occurs. For example, magnesiumwould corrode very quickly when coupled with gold ina humid atmosphere. But aluminum would corrodevery slowly, if at all, in contact with cadmium. Aflashlight battery is an example of galvanic corrosion

put to practical use. In Figure 3-5, the zinc batterycasing steadily corrodes, supplying a steady flow oelectrons, but only when the switch is closed. Whenthe switch is open, there is no corrosion becauseelectrons are not able to leave the zinc anode.

3-8.4. ANODE AND CATHODE SURFACE AREAThe rate of corrosion also depends on the size of theparts in contact. If the surface area of the corrodingmetal (the anode) is smaller than the surface area othe less active metal (the cathode), corrosion will berapid and severe. But, when the corroding metal islarger than the less active metal, corrosion will be slow

and superficial. For example, an aluminum fastener incontact with a relatively inert monel structure maycorrode severely, while a monel bracket secured to alarge aluminum member would result in a relativelysuperficial attack on the aluminum sheet (seeFigure 3-6).

3-8.5. PRESENCE OF ELECTROLYTES. Electricallyconducting solutions are easily formed on metallicsurfaces when condensation, salt spray, rain, or rinsewater accumulate. Dirt, salt, acidic stack gases, andengine exhaust gases can dissolve on wet surfacesincreasing the electrical conductivity of the electrolyte

thereby increasing the rate of corrosion.

3-8.6. ELECTROLYTE CONCENTRATION. In thesame way that metals can corrode when exposed todifferent concentrations of oxygen in an electrolytecorrosion will also occur if the concentration of theelectrolyte on the surface varies from one location toanother. This corrosive situation is known as aconcentration cell.

3-8.7. AVAILABILITY OF OXYGEN. When some of theelectrolyte on a metal surface is partially confined (suchas between faying surfaces or in a deep crevice), metain this confined area corrodes more rapidly than othemetal surfaces of the same part outside this area. Thistype of corrosion is called an oxygen concentration celor differential aeration cell. Corrosion occurs morerapidly than would be expected because the reducedoxygen content of the confined electrolyte causes theadjacent metal to become anodic to the metal surfacesexposed to the air.



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ANODIC - High Corrosion Potential

CATHODIC - Low Corrosion Potential

LithiumMagnesium Alloys

Zinc (plate)Beryllium

Cadmium (plate)Uranium (depleted)

Aluminum AlloysIndium

Tin (plate)Stainless Steel 430 (active)

Lead1010 Steel

Cast IronStainless Steel 410 (active)

Copper (plate)Nickel (plate)

AM 350 (active)Chromium (plate)

Stainless Steels 350, 310, 301, 304 (active)Stainless Steels 430, 410 (passive)

Stainless Steel 13-8, 17-7PH (active)Brass, yellow, Naval

Stainless Steel 316L (active)Bronze 220

Copper 110Stainless Steel 347 (active)

Copper-Nickel 715

Stainless Steel 202 (active)Monel 400Stainless Steel 201 (active)

Stainless Steels 321, 316 (active)Stainless Steels 309, 13-8, 17-7 PH (passive)

Stainless Steels 304, 301, 321 (passive)Stainless Steels 201, 316L (passive)

Stainless Steel 286 (active)AM355 (active)

Stainless Steel 202 (passive)Carpenter 20 (passive)

AM355 (passive)Titanium Alloys

AM350 (passive)Silver

PalladiumGold

RhodiumPlatinum

Carbon/Graphite

Figure 3-4 . Galvanic Series of Metals and Alloys in Sea Water



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3-8.8. TEMPERATURE. Higher temperatureenvironments tend to produce more rapid corrosion dueto accelerated chemical reactions and, in humidenvironments, higher concentration of water vapor inthe air. In addition, nightly drops in temperature cancause greater amounts of condensation, leading to

increased corrosion rates.

3-8.9. BIOLOGICAL ORGANISMS. Bacterias, molds,fungi, and other living organisms (some microscopic)can grow on damp surfaces. Once they are wellestablished, the area tends to remain damp, increasingthe possibility of corrosion. Their presence can causethe areas they occupy to have different oxygen andelectrolyte concentrations. In addition, acidic wastesare secreted, which cause corrosion.

3-8.10. MECHANICAL STRESS. Many alloys used inaircraft construction are sensitive to a form of corrosionknown as stress corrosion cracking. Manufacturingprocesses such as machining, forming, welding, or heatreatment can leave residual mechanical stresses inaircraft parts. The addition of in-service stresses toresidual stresses can cause corrosion to proceed morerapidly than would be expected in normal service.

3-8.11. LENGTH OF EXPOSURE. As time passesmetals naturally tend to corrode. In some cases, thecorrosion process occurs at the same rate, no mattehow long the metal has been exposed to the environmentIn other cases, corrosion can decrease with time, due tothe barrier formed by corrosion products, or increasewith time if a barrier to corrosion is being broken down

Relatively LittleCorrosive Attack

HeavyCorrosiveAttack

Aluminum Sheet(Large Anode)

Monel Sheet

(Large Cathode)

Aluminum Rivet

(Small Anode)

Monel Rivet(Small Cathode)

Figure 3-6. Effect of Area Relationship inDissimilar Metal Contacts

Figure 3-5. Galvanic Corrosion in a Flashlight Battery

e

e

e

e

e

e

e

e

Electrolyte

Electrolyte

ZincAnode

CarbonCathode

Direction of

Current Flow

Metal Ions Go Into SolutionFrom the Anode

+



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3-9. TYPES OF CORROSION. Corrosion iscatalogued and typed in many ways. Occasionally,different names are used for the same type of corrosion.The common types of corrosion are described below.

3-9.1. UNIFORM SURFACE CORROSION. Uniformsurface corrosion is probably the most common type ofcorrosion. It results from a direct chemical attack on ametal surface that proceeds uniformly over the entireexposed surface (see Figure 3-7). The metal graduallybecomes thinner and eventually fails. On a polishedsurface, this type of corrosion is first seen as a generaldulling or etching of the surface and, if the attack is

allowed to continue, the surface becomes rough andpossibly frosted in appearance. An example is theetching of metals by acids. The discoloration or generaldulling of metal created by exposure to elevatedtemperatures is not considered to be uniform surfacecorrosion. Coating/sealing the exposed surface willprotect it from this type of attack. Also, corrosive elementsmay be removed through air movement and drain holes.

3-9.2. GALVANIC CORROSION. Galvanic corrosionoccurs when different metals are in contact with eachother and an electrolyte, such as sea water. It is usuallyrecognizable by the presence of a buildup of corrosiondeposits at the joint between the metals. For example,aluminum skin panels and stainless steel doublers,riveted together in an aircraft wing, form a galvaniccouple if moisture and contamination are present.Figure 3-8 shows galvanic corrosion of magnesiumadjacent to steel fasteners. The potential for galvaniccorrosion is greatest when the two metals are wellseparated from each other in the galvanic series (seeFigure 3-4) and are in electrical contact.

3-9.3. PITTING CORROSION. Pitting is a form ofextremely localized attack that results in holes in themetal (see Figure 3-9). Pits can be isolated, or soclose together that they look like a rough surface. Pitsare often difficult to detect because of their small sizeand because they may be covered with corrosionproducts. Pitting is usually first noticeable as a white orgray powdery deposit, similar to dust, which blotchesthe surface. When the deposit is cleaned away, tinypits or holes can be seen in the surface. Most pitsdevelop and grow downward (in the direction of gravity)from a horizontal surface. Pitting failures are commonlycaused by electrolytes containing chloride or chlorine-

Figure 3-7. Surface Corrosion on Frequency Test Set

Figure 3-9. Variations in the Cross-SectionalShape of Corrosion Pits

* Shapes determined by microstructural orientation

Horizontal* Vertical*

Narrow &Deep

Elliptical

Subsurface Undercutting

Wide and Shallow

Figure 3-8. Galvanic Corrosion of MagnesiumAdjacent to Steel Fastener

ElectrolyteCorrosion Products

Magnesium Alloy(Anode)

SteelFastener(Cathode)



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containing ions (such as seawater). Stainless steelsare most susceptible to pitting damage, althoughaluminum, magnesium, and copper are often attacked(see Figure 3-10).

3-9.4. INTERGRANULAR CORROSION.Intergranular corrosion is an attack on the grainboundaries of the metal. A highly magnified crosssection of any commercial alloy (see Figures 3-11 and3-12) shows the granular structure of the metal. Itconsists of quantities of individual grains, each havinga clearly defined boundary, which chemically differsfrom the metal within the grain. Frequently the grainboundaries are anodic (tend to corrode more easily) tothe metal within the grain. When an electrolyte ispresent, rapid selective corrosion of the grainboundaries occurs. High strength aluminum alloys,which depend on precipitated phases of alloying

elements for strength, are particularly susceptible tointergranular at tack. Figure 3-13 shows howintergranular corrosion progresses in 7075-T6aluminum alloy adjacent to steel fasteners. In thisexample, the grain boundaries are anodic to both themetal grain and the steel fastener.

3-9.5. EXFOLIATION CORROSION. Exfoliation (seeFigures 3-14 and 3-15) is an advanced form ofintergranular corrosion where the surface grains of ametal are lifted up by the force of expanding corrosionproducts occurring at the grain boundaries. The liftingup or swelling is visible evidence of exfoliation corrosion.

Exfoliation occurs on extruded, rolled, wrought, andforged high strength aluminum and magnesium parts.This type of corrosion most often occurs on extrudedsections of metal and is found primarily in aluminumsheet around steel fasteners. Its prevention involvesseparating the aluminum and steel by a barrier, such aszinc-chromate primer or sealant.

3-9.6. CREVICE CORROSION. Crevice corrosion isone of the most familiar types of corrosion. Fieldexperience shows that this type of corrosion may occurin any crevice where a stagnant solution has pooled.Crevices are usually located at gasket surfaces, lap

joints, and under bolt or rivet heads. Crevice corrosionoccurs because the environment of the local area isvery different from the larger environment. As a result,the metal surfaces, even though they may be the samemetal, have different activities, and corrosion occursinside the crevice. This kind of corrosion can also occurwhen a surface is covered by a foreign material. Methodsto minimize crevice corrosion include closing the crevice

Figure 3-11. Cross-Section of 7075-T6 Aluminum Alloy

Figure 3-10. Pitting of an Aluminum Wing Assembly

Figure 3-12. Scanning Electron Micrograph of aCorroding Aluminum Surface



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by welding, sealant, or soldering, and use ofnonabsorbent gaskets (such as Teflon).

3-9.6.1. Oxygen Differential Cells. Electrolyte incontact with metal surfaces will normally containdissolved oxygen. An oxygen cell can develop at anypoint where the oxygen in the air is not allowed todiffuse into the solution, thereby creating a differencein oxygen concentration between two points. Typicallocations of oxygen differential cells are under eithermetallic or non-metallic deposits (dirt) on the metalsurface and under faying surfaces such as riveted lap

joints. Oxygen cells can also develop under gaskets,

wood, rubber, plastic tape, and other materials incontact with the metal surface. Corrosion will occur atthe area of low oxygen concentration (anode) as

illustrated Figure 3-16, View A. Alloys such as stainlesssteel, which owe their corrosion resistance to surfacepassivity, are particularly susceptible to this type ofcrevice corrosion.

3-9.6.2. Active/Passive Cells. Metals which dependon a tightly adhering passive film, such as the oxidefilm on corrosion resistant steel, are prone to rapidcorrosive attack by active/passive cells. The corrosiveaction usually starts with a deposit of dirt or salt which

Figure 3-14. Extreme Example of Exfoliation atEdge of Sheet

Figure 3-15. Exfoliation Adjacent to Fasteners

Figure 3-13. Intergranular Corrosion of 7075-T6Aluminum Adjacent to Steel Fasterner

Electrolyte EntersThrough Cracks inPaint Film Intergranualar

CorrosionPaint Film

7075-T6 Aluminum(Anode)

SteelFastener(Cathode)



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creates an oxygen differential cell. The passive film isthen broken in the area of the salt deposit and the moreactive metal beneath the passive film will be exposedto corrosive attack. This small anodic area will thencorrode rapidly due to the much larger area of thesurrounding cathode (passive film). The result is rapid

pitting of the surface, as illustrated in Figure 3-16,View B.

3-9.7. FILIFORM CORROSION. Filiform corrosion isa form of crevice corrosion which occurs on metalsurfaces having a thin (~4 mils) organic protectivecoating. It is recognized by its characteristic wormliketrace of corrosion products beneath the coating (seeFigure 3-17). Filiform corrosion is unusual because itonly affects surface appearance, but does not weaken

or destroy the base metal. Filiform corrosion occurswhen the relative humidity of the air is between 65 and90%, and the air temperature is between 70and 100FIt starts at breaks in the coating system, such asscratches and cracks around fasteners and seams, andproceeds underneath the coating, due to the diffusionof water vapor and oxygen from the air through thecoating (see Figure 3-18). Filiform corrosion can attack

Figure 3-16. Crevice Corrosion Mechanisms

Passive FilmProtects ExposedSurface

Electrolyte Low Oxygen Concentration(Corrosion Site)

High Oxygen Concentration

Pitting Corrosion

Foreign MaterialCreates Low OxygenRegion Which Preventsthe Re-Formationof Passive Film

Active Metal

A. Oxygen Differential Cell

B. Active Passive Cell

Figure 3-17.Filiform Corrosion Found Under

Paint Coating on a Magnesium Panel

Figure 3-18. Schematic of the Development of Filiform Corrosion on an Aluminum Alloy

CathodeOrganic

Coating

Break in

Coating

Base Metal

Anode



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steel, magnesium, and aluminum surfaces and maylead to more serious corrosion in some locations.Filiform corrosion can be prevented by storingequipment and aircraft in an environment with a relativehumidity below 65%, by using coating systems having

a low rate of diffusion for oxygen and water vapors, bymaintaining coatings in good condition (prompt touchuparound fasteners), and applying corrosion preventivecompounds (CPCs) when the coating is damaged.

3-9.8. EROSION CORROSION. Erosion corrosion isthe increase in the rate of attack on a metal due to theaction of a corrosive fluid against the metal surface.Generally the movement is rapid, and wear or abrasionoccurs with the corrosion. Erosion corrosion ischaracterized by grooves, gullies, waves, rounded holesand/or valleys in the metal surface. Metals that are soft(copper, lead) or metals that depend upon the

development of a protective surface film (aluminum,stainless steel) are susceptible to erosion corrosiondamage. Equipment exposed to moving fluids (e.g.heat exchanger tubing, pumps, propellers, impellers)are also susceptible.

3-9.9. STRESS CORROSION. Also called stresscorrosion cracking (SCC). Stress corrosion(Figure 3-19) is the intergranular or transgranularcracking of a metal caused by the combined effects ofconstant tensile stress (internal or applied) and corrosion.Internal or residual stresses may be produced by welding,cold working, forming, and heat treatment operations

during the manufacture of a part. Stresses remainconcealed in the part unless stress relief operationsare used. Other hidden stresses are induced in partswhen press or shrink fits are used and when slightlymismatched parts are clamped together with rivets andbolts. All these stresses add to those caused by applyingnormal loads to parts in operation. Stress corrosion isnormally localized and appears in the form of cracks.During SCC, the metal is unattacked over most of itssurface, while fine cracks progress through the interiorof the part. Cracking is generally perpendicular to theapplied stress. Metals have threshold stresses belowwhich stress corrosion cracking will not occur. Thisthreshold stress varies from metal to metal, is differentfor different tempers of the same metal, and is differentfor each of the three directions in which stress can beapplied. In aircraft, high strength steel parts (e. g.landing gear) and high strength aluminum parts areparticularly susceptible to stress corrosion.

3-9.9.1. Associated Hazards. Stress corrosion crackingis an extremely dangerous type of failure because it can

occur at stress levels far below the rated strength of ametal, starting from what appears to be a very minorcorrosion pit. This type of failure can be catastrophicand occur without warning. Parts can completely severin a split second or they can crack slowly. The rate ofcracking and the stress limit is very unpredictable inoperating service. For example, 7075-T6 aluminumalloy can fail by stress corrosion cracking when subjectedto a stress which is only 10% of its rated strength.

3-9.9.2. Causes. Specific environments have beenidentified which cause stress corrosion cracking ofcertain alloys. Salt solutions, sea water, and moist saltladen air may cause stress corrosion cracking of heattreatable aluminum alloys, stainless steels, and sometitanium alloys. Magnesium alloys may stress corrodein moist air. Stress corrosion can be prevented byplacing an insulating barrier between the metal and thecorrosive environment, or by applying protective coatingsand/or water displacing corrosion preventivecompounds. Stress relief operations during fabrication

Figure 3-19. Cracking (Typical of Stress Corrosion orCorrosion Fatigue)



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of parts will help, because it lowers the internal stresslevel of the part. Shot peening a metal increasesresistance to stress corrosion cracking by creatingcompressive stresses on the surface, which must beovercome by an applied tensile stress before the

surface sees any tension load. Changing the alloy (forexample, replacing an aluminum 7075-T6 part withone made from 7075-T73 or T76 alloy) can greatlyincrease resistance to SCC.

3-9.10. CORROSION FATIGUE. Corrosion fatigue(see Figure 3-19) is the cracking of metals caused bythe combined effects of cyclic stress and corrosion. Nometal is immune to reduction in its resistance to cyclicstress if the metal is in a corrosive environment. Corrosiondamages the metal by pitting, followed by crack formationin the pitted area due to cyclic stress. The crack ispropagated predominantly in the fatigue mode, in which

the rate of cracking is controlled by the stressconcentration in the main cross section, the physicalproperties of the metal, and the presence of corrosionproducts on the crack face. Fracture of a metal part dueto corrosion fatigue occurs when the remaining cross-sectional area is unable to carry the applied loads. Likestress corrosion, corrosion fatigue is normally localizedand appears in the form of cracks. The metal is generallyunattacked over most of its surface, while the crackprogresses through the part. Cracking is generallyperpendicular to the applied stress. Protection of allparts subject to cyclic stress is particularly important,even in environments that are only mildly corrosive.

Preventive measures include reducing the stress on thepart, using corrosion inhibitors, and applying a metalliccoating (e.g. chromium, cadmium, or ion vapor deposition(IVD) aluminum) to the part.

3-9.11. FRETTING CORROSION. Fretting corrosionoccurs at contact areas between materials under loadsubject to repeated vibration. The relative motion neededto produce fretting is extremely small (sometimes aslittle as 10-8 cm). The corrosion products increase thewear of the surface, and the wear exposes more baremetal surface to be corroded. The overall effect isgreater than the single effects of corrosion and wearadded together. Fretting has the general appearance ofgalling, in which chunks of metal are torn from thesurface with corrosion at the torn areas or ragged pits(see Figure 3-20). Although fretting corrosion can takeplace on any metal, aluminum, stainless steel, andtitanium alloys are most susceptible. These metalsdepend on an oxide surface film to inhibit furthercorrosion. With rapid movement under pressure at theinterface, the oxides are removed and rapid oxidation

occurs. Moisture does not appear to increase thecorrosion; in fact, it tends to slow down the reactionFretting corrosion is normally encountered in heavilyloaded static joints which are subject to vibration, suchas landing gear component attachment areas havinglug holes with slight press fits, slip fit bushings withvery close tolerance bolts passing through the bushingswing root access panels or wing-to-body fairings, andengine blade roots. Practical means of reducing frettingcorrosion include reducing the amount of relative motionat the surface, adding a lubricant at the interface toreduce friction and seal out oxygen, increasing thesurface hardness of the part, and increasing the overal

hardness of one or both contacting metals.

3-9.12. HOT CORROSION. Also call ed hightemperature oxidation. Corrosion in the absence owater can occur at high temperatures, such as thosefound in turbine engine combustors, turbine sectionsand afterburners. When hot enough, metals can reactdirectly with the surrounding gases, producing oxidescale (see Figures 3-21 and 3-22). Contaminantssuch as chlorides and sulfates, can accelerate the hocorrosion reaction by reducing the melting point of themetallic oxide and promoting its vaporization. Hightemperature ceramic coatings can reduce this type o

corrosion but are usually applied only by themanufacturer due to the highly specialized equipmenrequired for application.

3-10. METALS AFFECTED BY CORROSION. Nometal has perfect environmental integrity and is totallyresistant to corrosion. As a result, all metals will corrodesooner or later. The characteristics of corrosion onaircraft metals are summarized in Table 3-1. The

Figure 3-20. Fretting Corrosion



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following paragraphs discuss the corrosion

characteristics of commonly used aircraft metals.

3-10.1. ALUMINUM. Aluminum and aluminum alloysare the most widely used materials for aircraftconstruction. In addition to its uses in aircraft structure,aluminum and aluminum alloys are widely used inequipment housings, chassis, mounting racks,supports, frames and electrical connector shells.Aluminum is highly anodic, as evidenced by its positionin the galvanic series table. It is anodic to most othermetals, and, when in contact with them, galvaniccorrosion of the aluminum will occur. Aluminum alloysare subject to pitting, intergranular corrosion, andstress corrosion cracking. In some cases, the corrosionproducts of a metal in contact with aluminum arecorrosive to aluminum. However, the formation of atightly adhering oxide film offers increased resistanceunder mild corrosive conditions. The corrosion productof aluminum is a white to gray powdery material(aluminum oxide or hydroxide) which can be removedby mechanical polishing or brushing with abrasives(Figure 3-23). Therefore, it is necessary to clean andprotect aluminum and its alloys against corrosion.Since pure aluminum is more corrosion resistant thanmost alloys, aluminum sheet stock is often covered

with a thin layer of nearly pure aluminum called claddingor alclad. However, in a marine environment, allaluminum surfaces require protection. Cladding iseasily removed by harsh treatment with abrasives andtooling, exposing the more corrodible alloy surface.Chemical conversion coating, paints, and corrosionpreventive compounds are the main methods ofprotection.

3-10.2. ANODIZED ALUMINUM. Some aluminum

parts are protected with an electrochemically appliedoxide coating called anodize. Aluminum oxide film onaluminum is a naturally occurring protective film, andanodizing merely increases the thickness of the oxidefilm. When this coating is damaged in service, it can beonly partially restored by chemical surface treatment.Unnecessary destruction (e.g. nicks and scratches) ofthe anodized surface must be avoided.

3-10.3. MAGNESIUM. Magnesium alloys are thelightest structural metals used for aircraft and missileairframes. Magnesium alloys are used extensivelythroughout avionic systems as antennas, structures,chassis, supports, and frames (radar). Magnesium isalso used extensively for transmission and gearboxhousings. These alloys are highly susceptible tocorrosion when the metal surface is exposed to theenvironment without a protective finish. The corrosionproducts are white powdery snow-like mounds (seeFigure 3-24). The deposits have a tendency to raiseslightly and the corrosion spreads rapidly. When thewhite puffy areas are discovered, it requires prompttreatment or the corrosion will penetrate entirely throughthe structure. The natural oxide-carbonate film formedon magnesium alloys does not provide sufficient

corrosion protection even in the mildest environment.The rate of corrosion of a magnesium alloy increaseswhen the alloy is immersed in water or periodicallysubjected to moisture. Corrosion may also beaccelerated by dissimilar metal couples and whenconductive contaminants are dissolved in the water.Corrosion of magnesium alloys can be greatlydiminished by the use of the proper protective finish,such as magnesium conversion coating and paint.Some magnesium parts in current aircraft have beenoriginally protected by anodizing processes. Coatings

Figure 3-22. Hot Corrosion on Engine ComponentsFigure 3-21. Hot Corrosion on Fasteners



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Table 3-1. Effects of Corrosion on Metals

SYOLLA HCIHWOTKCATTAFOEPYT

ELBITPECSUSSIYOLLAFOECNARAEPPA

TCUDORPNOISORROC

yollAmunimulA sserts,noitailofxe,ralunargretni,gnittipecafruS

gnitterfdna,gnikcarceugitafdnanoisorroc redwopyargotetihW

yollAmuisengaM gnittipotelbitpecsusylhgiH ,sdnuomekilwons,yredwop,etihW

ecafrusnostopsetihwdna

woL&nobraCleetSyollA

)seires0008-0004(

;gnittipdnanoitadixoecafruSnoisorrocralunargretnidnaecafrus

)tsur(edixonworb-hsiddeR

sleetSsselniatS)seires004-003(

eniramnignittipemos;noisorroceciverCralunargretni;gnikcarcnoisorroc;stnemnorivne

004(noisorrocecafrus;)seires003(noisorroc)seires

,nworb,derasemitemos;ecafrushguoRniatskcalbro

yollAmuinatiT

detaeperrodednetxe;tnatsisernoisorrocylhgiHnitluseryamstnevlosdetanirolhchtiwtcatnoc

.seitreporplarutcurtss'latemehtfonoitadarged.tnemelttirbmeesuacnacslootdetalpmuimdaC

.erutarepmetwoltastcudorpnoisorrocelbisivoNevobapolevedsedixoecafrusderoloC

)C073(F007

sadesu(muimdaCgnitalpevitcetorpa

)leetsrofnoisorrocecafrusmrofinU

kcalbronworbottisopedyredwopetihwmorF.ecafrusehtfognilttom

)etalp(muimorhC stiperehwleetsfognitsursetomorp(gnittiP

)etalpniruccognitalpfogniretsilb;stcudorpnoisorrocelbisivoN

gnitfildnagnitsuroteud

yollAesab-lekciN)lenoM,lenocnI(

;seitilauqtnatsisernoisorrocdoogsahyllareneGretawaesnignittipotelbitpecsus

tisopedyredwopneerG

lekciNsselortcelE)gnitalpasadesu(

gnitalpecafrusfognikalfdnagnittiPnoisorrocsetomorptub,edorroctonseodlekciN

ehtniruccostiperehwlatemesabmunimulafognitalp

,yollAesab-reppoCeznorB,ssarB

noisorrocralunargretnidnaecafruS tisopedyredwopneerg-eulbroeulB

revliS ruflusfoecneserpnihsinratlliW mlifkcalbotnworB

dloG tnatsisernoisorrocylhgiH secafrusevitcelferfogninekradesuacstisopeD

niT htworgreksihwottcejbuS stisopedekil-reksihW

Figure 3-23. Aluminum Surface Corrosion Products Figure 3-24. Magnesium Corrosion Products



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of this type are thicker than those applied by immersionor brush on conversion coating. Anodized finishes cannot

be restored in the field. Care should be taken to minimizeremoval of these coatings.

3-10.4. STEEL. Ferrous (iron) alloys are used tomanufacture many aircraft components and massivestructures and assemblies in aircraft ground supportequipment, such as missile gantries, silo crib structures,frames and bodies of trailers and vans, and lesserstructural parts such as brackets, racks, and panels.Iron and steel are also used as component leads,magnetic shields, transformer cores, brackets, racks,and general hardware in avionic systems. If unprotected,ferrous alloy surfaces (with the exception of stainless

steels) are easily corroded in the presence of moisture.Corrosion of steel is easily recognized because thecorrosion product is red rust (Figure 3-25). Whenferrous alloys corrode, a dark corrosion product usuallyforms first. When moisture is present, this coating isconverted to red rust, which promotes further attack byabsorbing moisture from the air. Ferrous alloy surfacesof structures or assemblies are normally painted, orplated with nickel, tin, or cadmium and painted, toprevent corrosion.

3-10.5. STAINLESS STEEL. Stainless steels, orcorrosion resistant steels (as they are more properlydescribed) are alloys of iron containing large amountsof chromium and nickel. Stainless steels are used forgears, bearings, and high strength bolts, and formountings, racks, brackets, and hardware in avionicsystems. The main reason for the existence of stainlesssteels is their resistance to corrosion. Stainless steelsare much more resistant to common rusting, chemicalaction, and high temperature oxidation than ordinarysteels, due to the formation of an invisible oxide film, or

passive layer, on the surface of these alloys. Corrosionand heat resistance are the major factors in selectingstainless steels for a specific application. However, it

should be well understood that stainless steels are notthe cure-all for all corrosion problems, due to serviceconditions which can destroy the oxide film on theirsurfaces. Stainless steels are susceptible to crevicecorrosion and stress corrosion cracking in moist, saltladen environments. Exposure to saltwater can causepitting. The corrosion product of stainless steel is aroughened surface with a red, brown, or black stain.Corrosion treatment of stainless steel should be limitedto cleaning. Stainless steels can cause galvaniccorrosion of almost any other metal with which they arein contact if proper techniques of sealing and protectivecoating are not used. Stainless steels may be magnetic

or non-magnetic. The magnetic steels are identified bynumbers in the American Iron and Steel Institute (AISI)400 Series (e.g. 410, 430). These steels are not ascorrosion resistant as the non-magnetic steels, whichare identified by numbers in the AISI 300 Series (e.g.304, 316).

3-10.6. TITANIUM. Titanium and titanium alloys findnumerous uses in aircraft, engines, and missiles attemperatures up to 1000F (540C). Above 1000F,titanium readily absorbs gases from the surrounding airand becomes very brittle. Under certain conditions,chlorides and some chlorinated solvents may inducestress corrosion cracking of certain titanium alloys.Titanium and its alloys are highly corrosion resistant. Anoxide film forms on their surfaces almost immediatelyupon contact with air, which is extremely adherent tothe surfaces and provides a protective barrier. This isidentical to the way aluminum forms a protective oxidefilm on its surface. Even at temperatures approaching1000F, titanium retains its strength and corrosionresistance. When titanium is heated, oxides having

Figure 3-25. Steel Corrosion Products

Figure 3-26. Color Changes in Titanium Due to Heating



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different colors form on the surface (see Figure 3-26).A blue oxide coating will form at 700to 800F (370to425C), a purple oxide at 800 to 950F (425 to510C), and a gray or black oxide at 1000F (540C) orhigher. These are protective discolorations and shouldnot be removed. Since titanium is the less active member(cathodic) of most dissimilar metal couples, it can greatlyaccelerate corrosion of a dissimilar metal coupled to it.Electrical insulation between titanium and other metals

is necessary to prevent galvanic corrosion of the othermetal. Titanium in contact with a corroding metal canabsorb hydrogen and become brittle. Frequentinspection of such areas is required to insure thatinsulation failure has not allowed corrosion to begin.

3-10.7. CADMIUM. Cadmium is used as a coating toprotect steel hardware, such as bolts, washers, andscrews, and as plating on electrical connectors. It isused as a plating on high strength steel parts (e.g.landing gear) to improve resistance to corrosion fatigue.Cadmium may also used to provide a compatible surfacewhen a part is in contact with other materials. Cadmium,when coupled with steel, is anodic and protects the steelby galvanic action. Corrosion on cadmium is evidentby white to brown to black mottling of the surface.When cadmium plate shows mottling and cracks in thecoating, the plating is still performing its protectivefunction as an anodic material (see Figure 3-27). Thecadmium plate on iron or steel is still protecting thebase metal until signs of rust begin to appear. Eventhen, any mechanical removal of corrosion products

should be limited to metal surfaces from which thecadmium has been depleted. Care should be taken noto remove undamaged cadmium plate adjacent to thecorroded area.

3-10.8. CHROMIUM. Chromium is used as a protectiveplating. Chromium plating is also used to provide asmooth, wear-resistant surface and to reclaim wornparts. When corrosion resistance in a marineenvironment is required, a nickel under-coat is usedThe degree of protection is dependent upon platingthickness. Chromium forms a continuous oxide coatingthat can be polished to a high luster and still protect noonly itself but any underlying metal. Chromium coatingscontain cracks, and corrosion may originate at the basemetal below these separations. Figure 3-28 shows theresults of a failed chromium plate.

3-10.9. NICKEL. Nickel is important as a plating metalan additive to stainless steel, and a base for nickealloys. Pure nickel is used as an electroless coating andis subject to pitting corrosion. Flaking of the nickecoating can also occur when an underlying metacorrodes. When added to stainless steel alloys, thestress corrosion resistance increases with nickecontents above 10%. Nickel based alloys are used inhigh temperature areas (engines, afterburners), buthey are subject to hot corrosion attack and embrittlemenwhen sulfur containing gases are present.

3-10.10. COPPER AND COPPER ALLOYS. Coppeand copper-based alloys (brass and bronze) areconsidered corrosion resistant, with corrosion usuallylimited to staining and tarnish. Copper and copper-based alloys are generally used in avionic systems ascontacts, springs, leads, connectors, printed circuiboard (PCB) conductors, and wires. Generally, changesin surface conditions are not dangerous and shouldordinarily have no effect on the part. Copper corrosionis evidenced by the accumulation of blue or blue-green

Figure 3-27. Cadmium Plated Surface Conditions

Figure 3-28. Failed Chromium Plate



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corrosion products on the corroded part (seeFigure 3-29). Sometimes copper or copper alloysurfaces tarnish to a dull gray-green color (patina) andthe surface will remain relatively smooth. Thisdiscoloration is the result of the formation of a fine-grained, airtight copper oxide crust. This crust offersgood protection for the underlying metal in ordinarysituations. However, exposure of copper and copperalloys to moisture or salt spray causes the formation ofblue-green salts, indicating active surface corrosion,which should be removed. When coupled with mostmetals used in aircraft construction, copper is the lessactive metal and greatly accelerates corrosion of iron,

steel, aluminum, and magnesium. Examples are usuallyfound in electrical components and in areas wherecopper bonding strips or wires are fastened to analuminum chassis or structural components. Protectivepaint coatings are seldom required because of theinherent resistance of the metal. However, paint finishesmay be applied for decorative purposes or if the normaltarnish or green patina on the copper is objectionable.

3-10.11. SILVER. Silver is used as a plating materialover copper in waveguides, miniature andmicrominiature circuits, wires, contacts, high frequencycavities, tank circuits, and RF shielding. Silver does notcorrode in the ordinary sense, although it will tarnish inthe presence of sulfur. The tarnish appears as a brownto black film. The tarnish is silver sulfide and may ormay not be detrimental to circuit electr icalcharacteristics, depending on the application. Whensilver is plated over copper there can be an acceleratedcorrosion of the copper. This occurs through galvanicaction at pinholes or breaks in the silver plating. Oneexample of this is the deterioration of silver plated

copper wire. This problem is compounded becausethe wire insulation prohibits detection of breaks in thesilver plating until damage is extensive. This redplague is readily identifiable by the presence of abrown-red powder deposit on the exposed copper.

Silver plating over nickel plate does not exhibit the redplague phenomenon.

3-10.12. GOLD. Traditionally considered the bestcoating for corrosion resistance and solderability, goldis used on printed circuits, semiconductors, leads, andcontacts. Gold is usually applied in

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AVIONICS CORROSION PROGRAM AND CORROSION THEORY